1. Field of the Invention
This invention relates to a laser beam scanning type eye fundus camera, in which the fundus of an eye to be tested is illuminated by scanning a laser beam.
2. Description of the Prior Art
Currently, there are two ways of observing and taking a picture of an eye fundus using an eye fundus camera; one is that the picture of an eye fundus is observed and taken by the ordinary procedure using visible rays (hereinafter simply referred to as "ordinary method"), and the other is that the picture of an eye fundus is observed and taken by fluorescence (hereinafter simply referred to as "fluorescence method" or "picture-taking by fluorescence").
When a picture of the eye fundus is to be taken by using light of a wavelength for exciting the fluorescent agent to generate fluorescence, a person to be tested usually gets an injection of a fluorescent agent into his arm vein, for example, and the injected fluorescent agent is circulated through the blood vessel of his eye fundus. On the other hand, the eye fundus camera usually irradiates light of a wavelength for exciting the fluorescent agent to generate fluorescence toward the eye fundus so that fluorescence is emitted at the blood vessel portion of the eye fundus, and a filter for the use of fluorescence is inserted into an optical path of an observation picture-taking optical system in order to cut a reflecting light of a wavelength other than the fluorescence.
In general, when a fluorescent agent is injected into the arm vein of a person to be tested, it usually takes about 5 to 10 seconds before the fluorescence starts its circulation in the blood vessel of his eye fundus, and it takes some more time until the fluorescence will have been fully circulated in the blood vessel of his eye fundus. Therefore, the inspector can know the state of the blood vessel of the eye fundus by observing and taking a picture of any change of the blood vessel of the eye fundus with the passage of time by fluorescence. However, since the change of the blood vessel of the eye fundus by the fluorescence is comparatively short in period of time from when the change of the eye fundus is first taken place until when it is stabilized, the picture taking operation is usually started immediately before the appearance of the blood vessel of the eye fundus by the fluorescence.
On the other hand, when the blood vessel of the eye fundus is to be observed and taken a picture by fluorescence, the eye fundus camera is naturally required to be well focussed on the eye fundus. Moreover, since the eye fundus is not constant and is easy to move, the focus must be adjusted frequently during the fluorescent observation picture-taking.
However, it occurs that the eye fundus is not seen at all in the early stage of the picture-taking operation of the blood vessel by fluorescence because the fluorescent filter is located in the optical path as described. Furthermore, the production rate of the fluorescence is very small at the blood vessel of the eye fundus in the early stage of the appearance of the blood vessel. Therefore, in the foregoing state, much difficulty is experienced in adjusting the focus of the eye fundus camera. Because of the foregoing reasons, it is desirable that the focus of the eye fundus camera can be adjusted by observing the object under the ordinary light or visible rays when the blood vessel of the eye fundus is to be taken a picture by fluorescence. However, the problem is that under such circumstance as that a filter for the use of fluorescence is disposed in the optical path as described, the eye fundus camera is practically impossible to be well focussed under the ordinary light.
Furthermore, the positional relationship between the blood vessel and the remaining portion of the eye fundus is difficult to obtain accurately only by the fluorescent picture-taking of the blood vessel of the eye fundus as described. Therefore, when the positional relationship is to be obtained accurately, it is recommended that the object be observed and photographed under the ordinary light and photographed under fluorescence, and both pictures taken are overlapped one upon the other for comparison.
However, when a filter for the use of fluorescence is disposed in the optical path of the conventional eye fundus camera, a picture of the blood vessel of the eye fundus can be taken only by fluorescence and a picture thereof can not be taken under the ordinary light. On the contrary, when a picture can be taken under the ordinary light, a picture taken by fluorescence can not be obtained because a filter for the use of fluorescence is not placed in the optical path of the eye fundus camera.
As a consequence, when the relationship between an image of the blood vessel of the eye fundus by fluorescence and that of the remaining portion thereof is to be obtained accurately, an observation picture-taking operation by fluorescence and an observation picture-taking operation under the ordinary light must be effected alternately.
However, since the objective eye fundus is very difficult to be kept constant, when the picture-taking by fluorescence and the picture-taking under the ordinary light are carried out alternately, a time lag occurs in two pictures taken in accordance with the afore-mentioned different procedures, i.e., between the image of the blood vessel of the eye fundus and that of the eye fundus. As a result, although it is possible to compare the image of the blood vessel of the eye fundus and that of the eye fundus by placing them side by side, it is impossible to obtain the accurate positional relationship by overlapping both the images.